The present invention relates to portable percussive machines of the type comprising a housing with a cylinder therein, in which a reciprocating drive piston via a gas cushion in a working chamber repeatedly drives a hammer piston to impact on the neck of a tool carried by the housing, and in which the hammer piston at empty blows is displaced towards the bottom end of the cylinder past ports in the cylinder wall through which said gas cushion is relieved so as to inactivate the hammer piston.
Percussive machines of the above type are usually hand held and used primarily for chiseling or drilling, powered by a suitable motor. Particularly in the higher power range of such tools suitable for instance for breaking, there is accentuated the problem that, if during full power operation the tool unexpectedly happens to slip aside from or to meet a crevice in the object operated upon, the hammer piston often will make a sudden empty blow of such strength that metallic collision thereof against the bottom end can occur with resultant risk of damage. The excessive heat generated in case pneumatic braking is practiced is liable to weaken the hammer piston seal, and a leaking or worn piston ring is certain to worsen the harmful effect of powerful empty blows.